A developing device adopted in a copying machine, a printing machine, a facsimile, or other electro-photographic apparatus is generally arranged such that toner is supplied to a photoreceptor which carries an electrostatic latent image formed on a surface thereof, to develop (visualize) the electrostatic latent image into a toner image. Specifically, in such developing device, toner is sequentially supplied onto a surface of the developing roller in a circumferential direction by a supply roller, and the toner held on the developing roller is carried onto the photoreceptor by the rotations of the developing roller.
A toner layer formed on the developing roller is regulated by a blade provided on the downstream side of the supply roller in the rotation direction of the developing roller. At the same time, the toner is charged by friction with the blade (friction charge). The charged toner held on the developing roller is then carried to a section facing the photoreceptor provided on the further downstream side in the rotation direction by the developing roller to be supplied electro-statically to the electrostatic latent image formed on the surface of the photoreceptor, thereby developing (visualizing) the electrostatic latent image into a toner image. The resulting visualized toner image is transferred to a recording material by the transfer means, and is then permanently affixed onto the recording material with an application of heat and pressure by the fixing means.
Japanese Laid-Open Patent Publication 7-281473/1995 (Tokukaihei 7-281473/1995), published on Oct. 27, 1995), Japanese Laid-Open Patent Publication 7-295327/1995 (Tokukaihei 7-295327/1995), published on Nov. 10, 1995), and Japanese Laid-Open Patent Publication 9-6132/1997 (Tokukaihei 9-6132/1997), published on Jan. 10, 1997) disclose the method of charging toner containing photo chromic compounds etc., which are reactive to light having a unique wavelength, wherein the toner is charged as being by directly irradiated with light in the developing device.
Japanese Laid-Open Patent Publication 4-220657/1992 (Tokukaihei 4-220657/1992), published on Aug. 11, 1992, and Japanese Laid-Open Patent Publication 7-234536/1995 (Tokukaihei 7-234536/1995), published on Sep. 5, 1995) disclose the method of charging toner by utilizing the photo-chromic reaction with an application of light onto the toner.
According to the foregoing conventional structure of charging toner, the blade for regulating the thickness of a toner layer is used also for charging the toner by friction. Namely, according to the foregoing conventional structure wherein the toner is charged by the friction with the blade, it is necessary to make the blade in tight contact with the developing roller with an applied relatively strong contact pressure (F) to charge the toner to a predetermined amount of charge. As described, in the foregoing conventional structure, since a high contact pressure is applied to the toner by the blade, it is liable that the toner be damaged by the applied contact pressure.
For the energy budget in the friction charging method, the following can be said. The driving energy (Ek) of the developing roller is converted into the toner layer thickness regulating energy (Es) and the toner charging energy (Et) by the function of the blade. Here, a part of the driving energy (Ek) is consumed as a thermal loss energy (El). Here, depending on the resulting thermal loss energy (El), a problem may arise in that the toner is made softer which expedites such problem that the toner is damaged or fused onto the surface of the blade, which deteriorates the characteristic of the toner being charged by friction.
On the other hand, when adopting the method of charging toner containing the special photo chromic compounds by irradiating the toner with light, it becomes difficult to adjust toner components due to the photo chromic material contained in the toner. It is therefore desirable not to adopt such toner whose components are difficult to be adjusted.
Particularly, in recent years, development has been made to achieve improved toner in terms of energy conservation. Specifically, for example, an attempt has been made to reduce the fixing energy by lowering the softening temperature of toner, or to improve the tinting power by increasing the number of toner pigment sections (reducing the breaking resistance of the toner). However, such toner is not suitably applied in the conventional friction charging method as strong contact pressure or heavy thermal load is applied to the toner.
The present invention is achieved in view of the foregoing problems associated with the conventional toner charging method, and it is therefore an object of the present invention to provide a developing device which can prevent deterioration of toner, i.e., prevent the toner from being damaged, or fused onto the blade, to realize a more reliable developing operation, and particularly to provide a developing device which can adopt such toner with a lower softening temperature, or toner containing an increased number of pigments to improve the tinting power.